In the past, methods for hollow injection molding have been developed and established mainly to achieve light weight, good rigidity, reduction in a number of parts and unification of composing parts among relatively large molded articles such as interior and exterior finish parts of cars and exterior finish parts of televisions. However, the hollow injection molding has been recently noticed as a molding method which meets increasing demands for improvement in dimensional accuracy and unification of composing parts among relatively small molded articles such as a gear and a roller.
Resin rotators are widely used as a mechanical part in various fields such as ordinary machinery, precision machinery and electric and electronic equipment. Since resin rotators are good at moldability, have light weight and do not get rusty, demand for rotators made of various resins has been increasing and high accuracy has been also required of such rotators in these years. Usually, such rotators are preferably molded so as to unify their whole body including a shaft for transmitting 100% of power.
Heretofore, attempts have been made to mold a shaft-unified type rotator by ordinary injection molding methods. However, when a rotator is molded in a unified body according to these methods, partially thick-walled parts are often produced in the resultant molded article with enhanced deformation such as sink marks, warpage and deflection of the resin. Consequently, the rotator tends to deteriorate in its dimensional accuracy such as cylindricity and coaxiality.
For improving the conventional injection molding methods, Japanese Patent Application Laid-Open No. 208460/1993 and DE Patent Publication No. 3835964 disclose an improved injection molding method for preparing a hollow molded article. This method comprises injecting a molten resin so as to leave a non-filled part and charging a pressurized gas into the resin so as to spread the resin into the non-filled part with a mold capable of forming a resin roller in a body.
Since the mold used in this method has a charging port for a pressurized gas in a cavity, traces of the charged gas remain at the part where the supply pipe is furnished. Therefore, a process for cutting off the traced part is required after the molded article is taken out from the mold. Further, since a cavity is arranged in the mold so that the axis of the cavity may be horizontal, the wall thickness is hard to be uniform in a cross-section vertical to the axis of the molded article. Accordingly, the molded articles have had many problems such that they do not have certain functions of rotators, since the centers of gravity and the shaft of the molded articles do no agree and their dimensional accuracy is poor.
Further, Japanese Patent Application Laid-Open No. 108558/1995 discloses a method for preparing a rotator by hollow injection molding with a mold having a structure wherein a cavity is arranged so that the axis of the rotator may be vertical, and a gate and an entrance for a pressurized gas are equipped at the bottom of the cavity.
The mold employed in this method has an entrance for a pressurized gas at the intersection of the axis of the rotator and the bottom of the cavity. However, the gate for a resin is arranged apart from the entrance for the gas so that it is difficult to inject a resin into the cavity under a uniform pressure. Accordingly, the wall thickness of the resultant rotator becomes ununiform around the gate. Since the gate for a resin and the entrance for the pressurized gas is arranged at the intersection of the axis of the rotator and the bottom of the cavity, it is difficult to take out the resultant rotator from the mold, and sometimes the gas may leak from the entrance for the gas at molding, or the rotator may deform when it is taken out from the mold.